1. Field of the Invention:
This invention relates to a circuit of a signal level converter, and more particularly to a level converter circuit converting a TTL (Transistor-Transistor Logic)-level signal to an ECL (Emitter-coupled Logic)-level signal.
2. Description of the Related Art:
The TTL-ECL level converter in the prior art used a positive power source supplying a power voltage for driving a TTL logic circuit, for example +5 volts, and a negative power source supplying a power voltage for driving an ECL logic circuit, for example -4.5 volts or -5.2 volts, together with a grounding potential. An input TTL signal is applied to a cathode of an input diode. The signal at the anode of the input diode is in turn applied to a base of an input transistor which is biased by a resistor or resistors connected to the positive power source. The collector of the input transistor is connected to the positive power source directly or through one of the resistors. The emitter of the input transistor is grounded through three diodes on one way and connected to a base of a buffer transistor through two of the three diodes and a resistor on the other way. The buffer transistor is grounded at its collector and connected at its emitter to the negative power source through a resistor. The signal level at the emitter of the buffer transistor is compared by a differential amplifier with a reference voltage which is a threshold voltage of the ECL circuit. The differential amplifier is also driven by the negative power source. The comparison result is derived through an emitter follower as an ECL signal. The emitter follower is also driven by the negative power source.
When the TTL input signal has a low level, the capacitance component of the input diode is charged with a current flowing through the positive power source--the resistor or resistors connected between the positive power source and the base of the input transistor--the input diode--the grounding point. The charges charged in the capacitance component of the input diode are discharged through the input transistor, when the TTL input signal changes to have a high level. Here, since the emitter of the input transistor is grounded through the three diodes, but the base thereof does not have a direct connection to the grounding point, the input transistor has a large input impedance. Therefore, a large time period is necessary to completely discharge the capacitance component of the input diode. Thus, a large time delay appears on the ECL output signal, when the input signal changes from a low level to a high level.
Besides, the discharging current also flows through the resistor or resistors biasing the base of the input transistor to the positive power source. In this current path, the voltage at the base of the input transistor momentally rises to a voltage which is an added value of the raised voltage of the input signal and the precharged voltage of the input diode. At this moment, the input transistor is driven into a saturation condition. If the transistor is saturated, a charge-storage is generated in the transistor and a new time delay appears when the input signal changes from a high level to a low level.